1. Field of the Invention
The present invention relates to an optical fiber for optical amplification, having a core portion doped with erbium as an optical amplification substance, and to an optical fiber amplifier and an optical-fiber laser.
2. Description of the Related Art
In recent years, optical fiber amplifiers using erbium doped optical fibers (EDF), which are optical fibers having core portions doped with erbium (Er) as an optical amplification substance, are used for various purposes.
The EDFs used in the optical fiber amplifiers are preferably those in which occurrence of nonlinear optical phenomena inside is suppressed so as to prevent degradation in the quality of amplified light. For example, Japanese Laid-Open Patent Publication No. 2003-318473 discloses an EDF having a refractive index profile adjusted to be of a so-called W-type or α-type. According to Japanese Laid-open Patent Publication No. 2003-318473, by making this adjustment to the refractive index profile, an absolute value of a wavelength dispersion of the EDF is increased and an overlap integral between an Er distribution in a core portion and a mode distribution of light propagated through the core portion is increased, thus increasing an absorption coefficient of the EDF and decreasing the length of the EDF to be used. Consequently, occurrence of nonlinear optical phenomena such as four-wave mixing and cross-phase modulation in the EDF is suppressed.
However, because optical fiber amplifiers have been used for more diverse purposes in recent years, intensities of light input to the EDFs tend to become higher. When light input to such an EDF has a high intensity, nonlinear optical phenomena tend occur in the EDF. Therefore, EDFs in which the occurrence of nonlinear optical phenomena is further suppressed have been demanded.